1. Set the oven to 325. Remove brisket from the refrigerator and let it set at room temperature for about 15 minutes while prepping other ingredients.
2. To a large Dutch oven, add the oil and sear the brisket on all sides – about 2-3 minutes per side. Remove and set aside.
3. To the Dutch oven, add the onion, garlic, cumin, and ancho chile, sauteing until the onion is translucent – about 1 to 2 minutes. Turn off the heat, and add the red wine vinegar and the Dr Pepper, scrapping up any cooked bits at the bottom of the pan. Add the brisket fat side up back to the Dutch oven. Cover and cook in the oven for 3 to 4 hours or until the meat is fork tender. Check the brisket after about 2 hours, if the liquids at the bottom of the pan have dried up or are beginning to caramelize, add a little bit of water. Cover and continue to cook.
4. When the brisket is done, remove the Dutch oven from the oven and place on the stove to cool uncovered for about 30 minutes. Meanwhile, prep the taco fixings.
5. To remove the skin from the poblanos, rub the peppers in oil and hold over the flame of a gas range, turning the pepper every few seconds to blacken and blister evenly. Carefully place the pepper into a brown paper bag and let it cool – the bag will help trap steam and make the blistered skin easier to remove. Once the pepper is cool enough to handle, rub the skin with a paper towel to remove. Remove the stem, ribs, and seeds and slice into thin strips.
6. In a skillet set over medium-high heat, add a bit more oil and the onions, cooking for a minute or two. Add the poblano strips and cook for another minute. Add a few whole stalks of cilantro and a lime wedge or two, letting them char slightly. Turn off the heat and squeeze a lime wedge over the pan, tossing to combine.
7. After the brisket has set for about 30 minutes, transfer the meat to a bowl. Using two forks, peel off and discard the remaining fat layer and shred the rest of the meat.
8. Pour the remaining drippings from the Dutch oven into another small bowl, straining out any remaining fat. Thin the drippings using a bit more Dr Pepper. Pour a bit of this liquid over the shredded brisket and set the rest aside for serving.
9. To assemble a taco: take a warm tortilla, top with brisket, a spoonful or two of the sauteed onions and peppers, a few cilantro leaves, and another squeeze of lime. Serve with rice, beans, salsa, chips, beer, etc.